The invention relates to a method and an apparatus for generating a color reference for a print image to be printed for visualizing the reference on a true-color monitor.
In accordance with normal practice, a reference sample produced on a colored ink jet printer serves as a color reference when running a print job on a press, as for example on a web rotary press or on a sheet-fed press, where a printer compares the copies printed on the press with this reference sample. Such reference samples printed on a colored ink jet printer are referred to as a hard proof. It is further known from practice to visualize in addition to, or as a substitute for, the hard proof produced on the colored ink jet printer a color reference on a true-color monitor. The visualization of a color reference on a true color monitor is referred to as a soft proof. The present invention relates to a method for generating a color reference for an image to be printed for visualizing the reference as a soft proof on a true-color monitor.
The procedure in practice when generating a color reference for an image to be printed for visualizing the reference as a soft proof is that lower-resolution image data are generated from high-resolution raster data which are obtained for a print form exposure from page description data with the aid of a raster image processor, where the lower-resolution image data are converted into display data with the aid of a color management device which serve as color reference for the image to be printed and can be displayed as a soft proof on a true-color monitor. In practice, high-resolution raster data are accordingly first reduced in resolution and converted into image data, where the display data serving as the color reference are obtained from the image data.
The adoption, or use, of the raster data generated in the raster image processor for generating the color reference for an image to be printed is problematic insofar as the generation of the raster data from the page description data in the raster image processor takes place while using RIP correction functions to compensate for non-linear transmission characteristics of tone values or full-coverage values in the exposure of print forms. The RIP correction functions are not consistent but rather are subject to modifications or adaptations in order to take into account the changing composition of a print form or a changing exposure process for the print form. Consequently, based on such changing RIP correction functions, page description data existing for example as tone values can be reproduced or imaged differently in the raster data to be generated from them, for example as full-coverage values. Since, however, the raster data are the basis for generating the display data serving as the color reference for a soft proof visualization, the changing RIP correction functions can affect the display data such that the data are degraded in their quality with respect to the soft proof to be visualized. Until now no methods for generating a color reference for an image to be printed for soft proof visualization of the reference are known which take the above problem into account.
Starting with this, the problem underlying the present invention is to create a method and a device for generating a color reference for an image to be printed which improves the color reproduction of the image to be printed.
This problem is solved by the invention in accordance with FIG. 1. In accordance with an embodiment of the invention, the method comprises at least the following steps: a) an actual transmission characteristic is ascertained which is used in the raster image processor to generate corresponding raster data from the page description data of the image to be printed; b) the actual characteristic is compared with a specified reference transmission characteristic; on the basis of this comparison, a correction curve is calculated which is required to arrive at the specified reference transmission characteristic starting from the actual transmission characteristic; and c) the display data are determined using the calculated correction curve.
Using the method in accordance with the invention, it is provided for the first time to determine an actual transmission characteristic which is used in the raster image processor to generate the raster data from the page description data, and on the basis of a comparison between the actual transmission characteristic ascertained and a specified reference transmission characteristic to generate a correction curve, using which the display data serving as color reference are established. The actual transmission characteristic ascertained is dependent on the RIP correction function in the raster image processor so that, with the aid of the calculated correction curve, data changes detracting from the quality of the soft proof visualization in the display data are compensated for or discounted. The quality of a soft proof visualization can thereby be improved.
In accordance with an advantageous further development, the actual transmission characteristic is determined by evaluating a defined area of the raster data or the image such that full-coverage values or tone values are ascertained for this defined area of the raster data or the image data, where these full-coverage values or tone values ascertained for the defined area are compared with full-coverage values or tone values which result with known page description data for the defined image area and with a linear transmission characteristic when converting the page description data into linear raster data or linear image data and where the actual transmission characteristic is calculated on the basis of this comparison.
In the case of the defined area, it is preferably a control element configured as a constituent part of the image to be printed, which has at least one field with a specified full-coverage value or tone value and at least one field to identify the control element in the raster data or image data.
The apparatus in accordance with an embodiment of the invention comprises: a) a device which automatically determines an actual transmission characteristic which is used in the raster image processor to generate the raster data automatically from the page description data for the image to be printed; b) a device which automatically compares the actual transmission characteristic with a specified reference transmission characteristic, and on the basis of this comparison, a correction curve is automatically calculated which is required to arrive at the reference transmission characteristic starting from the actual transmission characteristic; and c) a device which automatically determines the display data using the calculated correction curve.
Preferred further developments of the invention can be found in the description following. Embodiments of the invention are explained in greater detail using the drawing, without being restricted thereto.